A medical institution has a radioscopic apparatus installed therein for acquiring fluoroscopic images of patients. A conventional construction of such a radioscopic apparatus will be described. A conventional radioscopic apparatus includes a top board for supporting a patient, a radiation source disposed above the top board, and a radiation detecting device (FPD) disposed below the top board. The radiation source and FPD are movable along the body axis of the patient M.
The construction of a radiation source 53 will be described specifically. As shown in FIG. 7, the radiation source 53 has a disk-shaped rotating anode 61 with a tapered edge. The rotating anode 61 is located in a hollow portion of a vacuum chamber 62 where a vacuum is maintained. A support shaft 63 rotatably supports the rotating anode 61. A cathode 64 is installed in a position opposed to the edge of the rotating anode 61, from which electrons E are emitted to an edge region of the rotating anode 61. At this time, a high voltage is applied between the rotating anode 61 and cathode 64. The electrons E discharged from the cathode 64 impinge on the edge region of the rotating anode 61, from which an X-ray beam B is emitted outward of the vacuum chamber 62. A radiation source of such construction is described in Patent Document 1, for example.
The voltage applied between the rotating anode 61 and cathode 64 is supplied from a voltage applying unit 67. And a rotating mechanism 65 which rotates the support shaft 63 is provided in order to rotate the rotating anode 61 relative to the cathode 64.
An input unit 80 inputs instructions of the operator, through which the operator can freely control the radiation source 53. A main controller 81 carries out overall control of the components of an X-ray tube.
Operation of such radiation source 53 will be described. As shown in FIG. 8, at the beginning when an emission of radiation is stopped, voltage V between the two electrodes 61 and 64 is 0.
When the operator instructs an emission of radiation through the input unit 80, rotation of the rotating anode 61 is started at this point of time TA, and the number of rotations R of the rotating anode 61 which was 0 at the beginning increases. At the same time, the voltage applying unit 67 first applies between the two electrodes 61 and 64 a minimum voltage VL so low as to cause no damage to the rotating anode 61 even if the latter remains still.
When rotation of the rotating anode 61 is started, the rotating anode 61 will be brought to a predetermined number of rotations RA in due time. However, at the point of time TA the rotating anode stands still, and it takes some time to reach the predetermined number of rotations RA. This time required is set to t1.
If the high voltage is applied between the two electrodes 61 and 64 before the rotating speed of the rotating anode 61 is enough, electron-incident portions of the edge region of the rotating anode 61 will be heated to excess, and thus a possibility of damaging the rotating anode 61. In order to prevent this, according to the conventional construction, when fluoroscopy is started from the state of the rotating anode standing still, the minimum voltage VL (e.g. 50 kV) is first applied to the two electrodes 61 and 64 at the point of time TA. And at the same time as the speed of the rotating anode 61 increases, the voltage applied to the two electrodes 61 and 64 is gradually increased. Finally, the voltage applied to the two electrodes 61 and 64 will become voltage VA (e.g. 80 kV) suitable for diagnosis. The period from when the voltage VL is applied to the two electrodes 61 and 64 until the voltage VA suitable for diagnosis is reached is set to E. An ABC (automatic brightness controller) 70 carries out this voltage control, which adjusts brightness of fluoroscopic images by changing radiation intensity automatically.
Thus, according to the conventional construction, when fluoroscopy is started at the point of time TA for starting an emission of radiation from the state of the rotating anode standing still, the minimum voltage VL is constantly applied to the two electrodes 61 and 64.
[Patent Document 1] Unexamined Patent Publication H9-213494